The invention relates to a system for recording and playing back an MPEG information signal in tracks on a record carrier, and specifically a record carrier of the Digital Video Cassette Recorder (DVCR) type.
An MPEG information signal comprises a succession or stream of transport packets, which includes a data compressed digital video signal and a corresponding data compressed digital audio signal (and sometimes data signals), for broadcasting purposes or for transmission via a cable network. The MPEG information signal is in the form of transport packets having either an equal length or a variable length in time. In both cases, however, a transport packet comprises 188 bytes of information, the first byte of which is a synchronization byte.
A transmission such as an MPEG information signal in the form for recording on and reproduction from a record carrier, such as a magnetic record carrier as a tape, require special measures to be taken in order to realize such kind of transmission via the known tape format.
Storing a packet sequence number has its advantages if an MPEG data stream is received having a constant bit or transport rate without any gaps between packets, and comprising a number of different video programs interleaved in the MPEG data stream. Generally, such data stream may have too high a bit rate for recording the total data stream on the record carrier. For example, the MPEG bit rate for cable transmission is 45 Mbps, whereas the record carrier typically records with a 25 Mbps bit rate. The recording arrangement now comprises a program selector for retrieving one or multiple programs from the MPEG data stream so as to obtain the MPEG information signal for recording. As information corresponding to only one program is included in a MPEG transport packet, such a program selector selects, which is per se known, only those transport packets from the MPEG data stream that comprise information corresponding to wanted program(s). That means that some packets of the original MPEG data stream received are deleted. Upon reproduction, however, a valid MPEG video signal in accordance with the MPEG standard, however now comprising only the wanted programs, must be regenerated or recreated. By a "valid" MPEG signal or transport stream is meant a stream that satisfies the following requirements:
1. The program clock reference (PCR) in the packet is OK. The PCR is, typically, a 33 bit value of a sample of the local clock in the transmitter encoder. The PCR is used for clock recovery so that in the decoder, the local clock can be sync'd to the encoder local clock. PA1 2. Accumulated change to each PCR through the network must be kept within the limit specified by MPEG. PA1 3. The decoder transport buffers do not overflow.
Such regenerated data stream should have the transport packets that were selected upon recording in the same order. Upon recording a sequence number can be added to each transport packet received, also for any packets that will be deleted. The sequence numbers of the packets that are selected and stored may be stored in the third block section of the signal blocks in which a transport packet is stored. Upon reproduction, a sequence of numbers is retrieved, where subsequent numbers will not necessarily be next higher numbers. In that situation one or more dummy packets must be inserted so as to regenerate the replica of the original MPEG data stream.
It will also be apparent that a reproducing arrangement will be needed which is adapted to each specific embodiment of the recording arrangement, so as to enable a reproduction of the MPEG information signal recorded on the record carrier.
The two related copending applications, whose full contents are incorporated herein by reference, describe such systems which solve a problem arising from the asychronous nature of a channel represented by the DVCR and the necessity for preserving the timing critical data incorporated in the MPEG transport stream so that it can be reconstituted as a valid MPEG information signal upon playback for reproduction on a conventional TV set. The systems described involve tagging transport packets of the MPEG data stream, before inputting to the channel, with timing information, and using the timing information at the output end of the channel to recreate the proper data timing. Various schemes are described for packing the timing information tags with each or a plurality of transmission units of the transport stream. Using this basic tagging mechanism, transport streams of various types can be recorded and played back without losing any of the information in the original transmission. Where the transport rate of the transport stream is unknown, or with gaps between the transport packets (i.e., bursty), or the transport rate changes, then the referenced related applications describe ways of handling such data streams.
When, on the other hand, the transport rate of the incoming transport stream is constant and unknown, the related applications also describe schemes for handling this situation. Thus, using a combination of Time-Of-Arrival (TOA) and Sequence-Of-Arrival (SOA) tagging as described in the related applications, an MPEG-2 transport stream of unknown but constant transport rate can be recorded and recreated on playback. In this case, however, there must not be any gaps between the transport packets at the input.